Possible software flow problem in serial_core

["Bollinger, Seth" <Seth.Bollinger@digi.com>]

Hello All,

We’ve recently run into a possible problem with software flow control
handling in the serial_core layer.

Here is the scenario:

1. Transmit from uart to remote device
2. Remote device sends us an XOFF
3. The tty layer receives the XOFF
4. stop_tty() - The uart transmitter is stopped (ops->stop_tx) just as the
serial_core ring is cleared (this could trap a few bytes in the fifo).
5. Remote device sends us an XON
6. The tty layer receives the XON
7. start_tty() - However, the serial_core ring is empty, so the call to
start the uart transmitter (uart_start:ops->start_tx) is skipped

Any window, however small, could leave bytes stuck in the transmitter
forever -- particularly if there will be no further transmission until
receiving a response.

I can't find any functionality in the drivers that accounts for this
possibility.  Can you help me find how Linux serial drivers manage this
eventuality?

Thanks,

Seth